Objective The aim of this randomized controlled trial was to investigate the effect of an Internet‐delivered cognitive behaviour therapy (iCBT), which incorporated emotion‐regulation and conflict‐resolution techniques, on intimate partner violence (IPV). Another aim was to test the theoretical underpinnings of the treatment model using mediation analysis. Method Sixty‐five participants with aggression problems in intimate adult relationships were recruited from the community and were randomly assigned to iCBT or to a monitored waitlist control. Participants were assessed with standardized self‐report measures of IPV or aggression (Multidimensional Measure of Emotional Abuse, Revised Conflict Tactics Scale, and Aggression Questionnaire), relationship quality (Dyadic Adjustment Scale), anxiety or depression symptomatology (Patient Health Questionnaire; Generalized Anxiety Disorder Screener), at pretreatment, posttreatment (8 weeks), and 1‐year follow‐up. Process variables (subscales of Dysfunctional and Emotional Regulation Scale and Anger Rumination Scale) were assessed weekly over the active treatment phase. Results Robust linear regression analysis of all randomized participants showed significant treatment effects on emotional abuse relative to control at postassessment. Mediation analysis using growth curve modeling revealed that the treatment effect was partially mediated by changes in emotion‐regulation ability. Controlled effects on secondary outcomes were also observed. Analyses of uncontrolled effects indicted that gains on IPV were maintained at 1‐year follow‐up. Conclusions iCBT focusing on enhancing conflict‐resolution skills and emotion‐regulation ability has the potential to reduce IPV among self‐recruited individuals with mild forms of abusive behaviour in intimate relationships. Emotion‐regulation ability is potentially a key therapeutic process of change. Key Practitioner Message Internet‐delivered clinician‐guided cognitive behaviour therapy is a viable treatment option for reducing intimate partner violence among self‐recruited individuals with mild forms of abusive behaviour. For persons who display patterns of frequent and severe violence, other treatments are most likely needed. Emotion‐regulation training is potentially a key therapeutic component that ought to be incorporated in interventions targeting IPV.